NOTHING DATES faster than a biography, replete as they always are with the prejudices of their era. Now the consensus seems to be that we need new ones every 20 years or so - or else become victims of yesterday's thinking. But some figures get biographies every five years, implying either indulgence or a succession of redundancies. For the umpteenth time, we have another Byron biography, and yet again it contains no really new information - just a shameless re-editing of well-worn facts for the tastes and caprices of 1999.

Biographies have played a subtle but distinctive part in the development of Lord Byron's mystique. The poet's close friend, John Cam Hobhouse, scrawled revealing marginalia all over his copy of the first Byron biography, by Thomas Moore. This information has since played a key role in untangling much of what was hidden in Byron's public life - especially his homosexuality. The unravelling of all the secrets of Byron's life, undertaken by biographers this century with access at last to family papers, has led to an almost total re-evaluation of his work.

But biographers must have their axe to grind. Benita Eisler has already been criticised for an attempt to characterise Byron as a sexually abused child who went on to become a serial abuser of children: a slab of crude reductionism, were it true. One academic has attacked her for having a poor historical sense and for giving scant attention to Byron's poetic output - a charge that must be especially galling since Eisler complains "no 20th century biographer has troubled to examine his art". She obviously considers that she has done so.

To be fair, I think Eisler does try, on the whole, to avoid a crass, late-20th century gloss on the thorny subject of child abuse. Even so, she seems much more agitated by Byron having sex with young girls than young boys, giving quite unreasonable credence to Lady Byron's crazed jotting about the poet trying to rape 11-year-old Charlotte Harley. All biographers worth their salt accept that Lady Byron went to incredible lengths to gather with vindictive rage often dubious evidence against her estranged husband. As to Byron's succession of page boys, choirboys and Greek lads, Eisler seems as unconcerned and urbane as, say, Gore Vidal might be.

Her Byron is a snobbish, depressive, anorexic whirlwind of a man, rarely in control of his own life and always on the run from something (usually women). Since Byron's motives are inevitably complex, and so can be read either way, my feeling is that she tends to come down on the side of scepticism. She assumes he often acts from malice or insecurity. I think she is unfair. And although Eisler happens to be the author of a work on the class system in America, her take on Byron's aristocratic status (and the way he deals with it) is incredibly flat-footed.

As to Eisler's style, it is restrained and lofty without recourse to wit. She has that infuriating tendency among modern biographers to boil off primary sources almost before our noses, packing each paragraph with (too many) quotes. But quotes can come out of context and besides, the footnote is a far more glorious way of backing up assertions.

Eisler's footnotes are meagre; one in particular highlights that charge of poor historicity. In order to paint a picture of sexual depravity at Harrow during Byron's time, she quotes a lurid and famous description by AJ Symonds that is more Scum than Tom Brown's Schooldays - orgies, rapes, younger boys referred to as "bitches". But this refers to the situation at Harrow in 1854 - 53 years after Byron first went there. Knowing that she's pulling a fast one, Eisler vainly protests "there is every reason to suppose that what was common practice in Victorian Harrow had begun during the Regency".What reasons, pray, are those? Since when was 1801 "the Regency"?

It would be very hard to write a boring biography of Byron, and the story fairly rattles along: the troubled youth; the years of fame; scandal; decline; and Venice. But coming in at over 700 pages, this book neither has the gravitas of the definitive three-volume Leslie Marchand biography, nor the rude spriteliness of later revisionist studies - for example, the excellent Byron and Greek Love by Louis Crompton, which finally blew the lid on the homosexuality. Rather than read Eisler, the Byron novice would be far better seeking out these two works, and polishing them off with the Selected Letters.

Eisler's final judgement that Byron is all things to all people, claimed by everyone from queer theorists to Greek nationalists, and therefore ultimately a mystery, is true on one level. But it's also an admission of failure to identify the essence of the man. I felt this biography was neither a love affair nor a duel - and, with Byron, it must be one or the other.